Click Restore button, it will start restore the image into the USB drive.

NOTE: The whole process will take about half an hour. Right at the end, you might get an error message: "Could Not Restore - Invalid Argument". You can simply ignore it. The image should be restored to the USB drive successfully.

Method 2: Use Lion Diskmaker

Make sure your USB disk's partition type is GUID Partition Table, if not, re-partition your USB as described in Method 1 using Disk Utitlity. Otherwise, the disk made by Lion Diskmaker may not be able to boot.